Buildings are constructed primarily on concrete slabs which are dimensionally stable or on suspended wooden subfloors, including particleboard and plywood. Although the suspended wooden subfloors are less expensive than concrete flooring, the suspended wooden subfloor is less stable than concrete when subjected to humid summer/dry winter seasonal changes. Self-induced tension floor coverings have been developed for these less stable subfloors.
Deichert et al. U.S. Pat. No. 3,464,178, which is incorporated by reference, teaches a method of installing resilient sheet flooring having a porous backing and a thermoplastic resin layer which shrinks in both the machine direction and across machine direction by bonding the backing layer of the sheet flooring to the perimeter of the room.
Evans U.S. Pat. No. 3,990,929, which is incorporated by reference, teaches methods of manufacturing and installing self-induced tension flooring or surface covering by manufacturing the structure on a thermally dimensionally stable backing which is removed just prior to installation at the job site. The surface covering is secured at its perimeter only before the stresses therein are relieved
Evans U.S. Pat. No. 4,159,219 teaches a method to produce an unbacked tension flooring by the specific design of two or more thermoplastic layers constructed such that when rolled, the outward facing layer elongates and the inward facing layer is compressed. Upon unrolling and placing flat, the elongated layer overcomes the compressed layer and thus a length greater than the original length before roll-up is generated. This elongation is defined herein as "roll-up growth". On securing the surface covering only at its periphery shortly after unrolling, the tendency of the surface covering to return to its original dimension, i.e., its elastic memory, creates a self-induced tension therein.
Greiner, Jr. et al. U.S. Pat. No. 4,135,675 shows a paper carrier stripping method and stripping apparatus which prevents uncontrolled tension from the necessary procedure of removing a strippable carrier from the structure before placing into a roll upon a windup stand.
Installation of these elongated tension floors by attachment at their periphery has produced one of the most reliable installation methods to accommodate fluctuating dimensional changes of unstable wood subfloors in today's environment.
The prior art tension floors have included a thermoplastic base, a foamed thermoplastic layer and a wear layer. Interest has been expressed in a high-end, rotogravure, tension floor covering. Such a high-end product would have a thicker gauge such as 0.090 inches. Such a tension floor covering could be produced by merely increasing the thickness of the foam layer. However, it is also desired to improve the durability of the floor covering to justify the designation as a high-end product. Further, as the thickness of the foam layer is increased beyond about 25 to 30 mils, the chemically embossed images become less sharp. This is most likely due to the foam growth-controlling agent, such as inhibitor, accelerator or blowing agent, diffusing excessively in the foamable layer prior to foaming.
It is also desired to produce a rich appearance by using a platelet containing layer in the optical surface of the floor covering. The optical surface is defined to be the region of the floor covering which is visible. This region extends from the exposed surface into the floor covering and includes the transparent or translucent wear layer, if any, and at least a portion of the platelet containing transparent or translucent layer.
Such surface coverings are disclosed in copending commonly owned, U.S. application Ser. No. 344,708 now U.S. Pat. NO 5,223,322, which is incorporated by reference, and Wang et al. U.S. Pat. No. 4,863,782.
Various patents such as Mawson et al. U.S. Pat. No. 4,172,169; Brown U.S. Pat. No. 4,138,521; and Faust et al. U.S. Pat. No. 3,978,258 disclose floor coverings in which two foam layers are separated by a decorative coat of solid plastic or a non-woven tissue containing vinyl substrate. However, there is no suggestion of using such structures in a tension floor. As described in Witman U.S. Pat. No. 3,870,591, which discloses a non-foamed poly(vinylchloride) layer containing a crosslinked polymer between two foamed poly(vinylchloride) layers, such layers are included to improve dimensional stability which is the antithesis of the properties desired in a tension floor.
Nairn et al. U.S. Pat. No. 3,293,108, Palmer et al. U.S. Pat. No. 3,399,106 and Wang et al. U.S. Pat. No. 4,863,782 provide good summaries of chemical embossing techniques. While Nairn et al. discloses applying an inhibitor to the supporting base or underside of the resinous layer, none discloses or suggests applying the foam growth-controlling agent to the lower surface of the upper of two foamed layers or applying the foam growth-controlling agent to both foam layers of a dual foam layer structure.